Retina & Visual Science · Patient Series
Three cone types.
One missing or shifted pigment.
A different world of colour.
Colour vision deficiency is not seeing in black and white. It is a retina that builds colour from two cone signals instead of three, or from three that sit too close together. Here is how it works, why it runs through families the way it does, how it is tested, and what glasses and gene therapy can honestly offer.
ancestry (red-green)
same ancestry
normal vision
Section 01 — How Colour Is Built
The eye does not see colour.
It compares three signals.
Lose one, and the comparison collapses.
The retina has no "red detector" or "green detector" in the simple sense. It has three families of cone photoreceptor, each containing a pigment that responds to a broad band of wavelengths, and the brain infers colour from the ratio of their responses. Colour vision deficiency (CVD) is what happens when that ratio can no longer be computed properly.
The three cone types are named by the part of the spectrum where they are most sensitive: long-wavelength (L) cones peaking at roughly 560 nanometres, medium-wavelength (M) cones near 530 nm, and short-wavelength (S) cones near 420 nm. Microspectrophotometry of human cones by Bowmaker and Dartnall in 1980 placed the peaks close to these values. Because the L and M curves overlap heavily, the difference between them is small and precious — it is the entire basis of red-green discrimination.
In 1986, Nathans and colleagues identified the genes for the three cone pigments: the L and M pigment genes (now called OPN1LW and OPN1MW) sit side by side on the X chromosome, and the S pigment gene (OPN1SW) sits on chromosome 7. That single piece of genetics explains almost everything about who inherits colour blindness and why.
Color blindness, properly called colour vision deficiency, means one of the retina's three cone types is missing, or its pigment is shifted so close to another that the brain gets a weaker colour signal. Most cases are inherited and red-green, because the L and M pigment genes are on the X chromosome, which is why men are affected far more often than women. People with CVD still see colour, but confuse particular pairs of colours. It is not a disease that progresses when inherited, it cannot currently be cured, and tinted glasses alter appearance without restoring normal colour vision. New or one-sided colour loss later in life is different and needs examination.
Section 02 — Interactive
Watch the cone curves
move, merge and vanish.
Each ribbon below is one cone type's sensitivity across the visible spectrum. Step through the main types of colour vision deficiency and watch what happens to the gap between the L and M curves — the gap that red-green discrimination depends on.
The model makes one thing visible that a description struggles to convey: in the anomalous trichromacies, nothing is missing. All three cones are present; the problem is that two of the curves sit too close together, so the difference signal between them is small. That is why severity varies so widely within "the same" diagnosis — a small shift gives a mild deficiency that may go unnoticed for decades, while a large shift behaves almost like a missing cone. In the dichromacies (protanopia, deuteranopia, tritanopia), one curve is simply gone, and colours that differ only along that axis become indistinguishable.
Section 03 — Inheritance
Why it skips generations
and favours sons.
Red-green colour vision deficiency is X-linked recessive. A man has one X chromosome, from his mother; if its L or M gene is altered, he has nothing to compensate. A woman has two, so she is usually affected only if both copies are altered — and much more commonly she is a carrier with normal or near-normal colour vision.
| Parents | Each son | Each daughter |
|---|---|---|
| Carrier mother, unaffected father | 50% chance affected | 50% chance carrier, usually unaffected |
| Unaffected (non-carrier) mother, affected father | Not affected through the father | All carriers |
| Carrier mother, affected father | 50% chance affected | 50% chance affected, 50% carrier |
| Affected mother, unaffected father | All affected | All carriers |
This is the classic "grandfather to grandson" pattern: an affected man passes his X to every daughter, who become carriers, and each of her sons then has an even chance of being affected. Tritan defects behave differently because the S-pigment gene is on chromosome 7 — inherited tritan deficiency is autosomal dominant, affects men and women equally, and is rare.
How common is red-green deficiency? Birch's 2012 review of worldwide prevalence data reported roughly 8% of men and about 0.4–0.5% of women in populations of Northern European ancestry, with lower reported rates in many other populations, including several African, Asian and Indigenous groups. The US National Eye Institute uses the rounded figures of up to 1 in 12 men and 1 in 200 women. Rates in any given country depend on ancestry mix, and studies use different tests, so treat national figures as approximate.
ancestry with red-green CVD
two on the X chromosome
Ishihara edition
Section 04 — Testing
Screening finds it.
Grading it takes
a different test.
Most people discover their colour vision deficiency through a book of dotted plates at school, a driving or job medical, or a family member's comment. Those plates are good screening tools, but they answer only part of the question.
| Test | What it does | Limits |
|---|---|---|
| Ishihara plates | Pseudoisochromatic dot figures; fast, cheap, sensitive screen for red-green deficiency | Does not test tritan defects; does not reliably grade severity |
| HRR plates | Pseudoisochromatic plates that screen red-green and blue-yellow defects and give a rough severity grade | Still a screen; borderline results need confirmation |
| Farnsworth D-15 | Arrange 15 coloured caps in order; the crossing pattern shows the confusion axis (protan, deutan, tritan) | Mild deficiencies often pass; designed to separate moderate-to-severe from mild |
| Farnsworth-Munsell 100 Hue | Fine hue discrimination across 85 caps; quantifies total error and axis | Time-consuming; performance affected by attention and lighting |
| Anomaloscope | Match a red-green mixture to a yellow (Rayleigh match); classifies dichromat vs anomalous trichromat precisely | Specialist instrument, not available everywhere |
Lighting matters for every one of these tests: plate and cap tests are standardised for daylight-equivalent illumination, and testing under warm indoor bulbs or on an uncalibrated phone screen can produce misleading results in either direction. Online colour blind tests are useful as a nudge to get properly checked, not as a diagnosis.
Section 05 — Acquired Colour Loss
When colour vision changes
in adult life, it is a symptom.
Inherited CVD is present from birth, affects both eyes equally and does not progress. Colour vision that changes later — especially in one eye — is a different matter. It is a sign that something along the visual pathway is not working normally.
Any new, one-sided, or rapidly changing colour vision — particularly with blurred vision, pain on eye movement, or a dimmer view in one eye — should be examined by an eye specialist promptly rather than put down to inherited colour blindness.
Section 06 — What Actually Helps
Glasses, lenses, apps,
gene therapy: an honest ranking.
There is no cure for inherited colour vision deficiency. There are tools that change how colours look, and strategies that make daily life easier. They are not the same thing, and marketing often blurs the line.
Illustrative editorial summary of how established each approach is for everyday use — not measured effect sizes. See References for the source studies.
Notch-filter glasses block a narrow band of wavelengths where the L and M curves overlap most, which exaggerates the difference between some reds and greens. Some wearers describe colours as more vivid. But when Gómez-Robledo and colleagues tested them in 2018 on red-green deficient observers, the glasses did not produce meaningful improvement on standard colour vision tests, and their effect resembled that of an ordinary tinted filter. A single tinted contact lens in one eye works by creating a brightness difference between the eyes, which can help with some plate tests without giving genuine colour discrimination.
Gene therapy is the one approach that targets the cause. In 2009, Mancuso and colleagues added a human L-pigment gene to the retinas of adult male squirrel monkeys, which are naturally red-green dichromats, and the animals gained trichromatic colour discrimination. It was a striking result, because it showed the adult brain can use a new cone signal. It has not yet become a treatment for people; gene therapy work in humans has focused on more severe conditions such as achromatopsia.
Section 07 — School, Work & Daily Life
The cost of not knowing
is usually higher than the deficiency.
Most people with colour vision deficiency live full lives without much difficulty. The problems tend to come from not knowing — a child marked wrong on a colour-coded worksheet, or a young adult who discovers a career requirement only at the medical examination.
An eight-year-old keeps colouring maps and graphs "wrong" and is assumed to be careless. A school screening with Ishihara plates flags a red-green deficiency, and an anomaloscope later confirms deuteranomaly. Nothing about his eyes needs treatment. What changes is the environment: the teacher labels colours in text as well as colour, uses patterns on charts, and the family knows early that some careers have colour vision standards, so it is a planning conversation rather than a surprise at eighteen.
Occupational colour vision standards exist in fields where colour-coded signals carry safety meaning — aviation, maritime and railway work, some armed forces and police roles, and electrical trades among them. The specific standard, and which tests are accepted, varies by country and by role, and some services now use more task-relevant tests than plates alone. Anyone with a known deficiency considering one of these careers should check the current requirements with the licensing body before committing to training.
Section 08 — FAQ
Frequently asked questions
about color blindness.
Most color blindness is inherited. Normal colour vision relies on three types of cone cell in the retina, each with a light-sensitive pigment tuned to long (L, 'red'), medium (M, 'green') or short (S, 'blue') wavelengths. In inherited red-green deficiency, the gene for the L or M pigment is missing or altered, so one cone type is absent or its sensitivity is shifted too close to the other. Colour vision can also be lost later in life from eye or nerve disease, cataract, or certain medicines; that is called acquired colour vision deficiency.
The genes for the red (L) and green (M) cone pigments sit on the X chromosome. Men have one X chromosome, so a single altered copy is enough to cause red-green deficiency. Women have two, and a normal copy on the other X usually compensates, so women are most often unaffected carriers. In populations of Northern European ancestry, roughly 1 in 12 men and about 1 in 200 women have red-green colour vision deficiency; reported rates vary between populations.
Not at present. There is no approved treatment that restores missing cone function in inherited colour vision deficiency. Gene therapy restored trichromatic colour vision in adult squirrel monkeys in a 2009 study, but this has not become a human treatment. Tinted glasses and lenses can change how some colours look and may help with specific tasks, but they do not create the missing cone signal and do not produce normal colour vision.
They change contrast between some colours, which some wearers find pleasant or useful, but controlled testing has not shown that they restore normal colour vision. A 2018 study in Optics Express found that notch-filter glasses did not meaningfully improve performance on standard colour vision tests in red-green deficient observers. They should be thought of as a filter that alters appearance, not a correction, and they will not help someone pass an occupational colour vision standard honestly.
Screening usually starts with pseudoisochromatic plates such as the Ishihara test, which detect red-green deficiency quickly but do not test blue-yellow (tritan) defects. Plates such as the HRR test include tritan screening. Arrangement tests such as the Farnsworth D-15 and the Farnsworth-Munsell 100 Hue grade severity and type, and the anomaloscope, which matches a mixed red-green light to a yellow, is the reference method for classifying red-green deficiency precisely.
Yes. The natural lens yellows and darkens with age and cataract, filtering blue light, so colours can gradually look duller or more yellow without the person noticing. After cataract surgery, many patients notice that whites look whiter and colours look bluer or brighter, because the clear artificial lens transmits more short-wavelength light. A new or one-sided change in colour vision is different: it can signal optic nerve or macular disease and should be examined promptly.
References & Evidence Base
Peer-reviewed
citations.
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